Metal oxide pigments are used widely for colorants in the paint, plastics and ceramics industries and are principally known for their chemical, thermal and weathering stabilities. Stable metal oxide pigments are produced by calcining simultaneously an intimate mixture of oxides or oxide precursors of primarily transition metals. In some cases the major component is a white pigment such as titanium dioxide which accepts minor additions of intensely colored metal ions to produce pastel colorants. A large number of such systems is disclosed in U.S. Pat. No. 3,022,186.
A previously known commercial pastel yellow pigment was made as a solid solution by incorporating antimony and nickel, as oxides or oxide precursors, as guest components in a titanium dioxide host lattice. It should be noted that nickel by itself (without the addition of antimony) does not form the yellow color complex in the titanium dioxide but results in a pale color.
However, much emphasis has been made of the suspected health hazards of pigments containing certain metals such as antimony, arsenic, bismuth, cadmium, selenium mercury and soluble barium. Consequently, it is desirable to produce an antimony-free pigment having the same qualities as the peviously known antimony-containing pigment.
While the same basic pigment compositions are used by such varied industries as the paint, plastics and ceramics industries to color a variety of media ranging from complex organic polymers to glass, it is necessary to adjust the physical properties of each pigment to meet the special specifications for its successful commercial application. For example, a ceramic pigment is usually composed of relatively coarse particles to minimize dissolution in the strongly alkaline glazes thereby to maintain color intensity. Conversely, the paint and plastics industries prefer pigments in a relatively finely divided state to permit easy dispersion and optimize such properties as gloss, brightness, strength and opacity,
The standard preparation of metal oxide pigments consists of calcining an intimate mixture of metal oxides or oxide precursors. Color properties develop from the formation of solid solutions. The above-noted patent U.S. Pat. No. 3,002,186 discloses solid solutions resulting from a large number of combinations of metal oxides or fluorides as guest components in a number of host lattices, principally rutile titanium dioxide. Examples 3, 8, and 9 of that reference show specifically various yellow solid solutions incorporating each of a combination of zinc oxide and tungsten oxide, a combination of nickel oxide and tungsten oxide and a combination of nickel oxide, tungsten oxide and sodium fluoride in a titania lattice. Pigments prepared as disclosed therein possess the desired color properties but are deficient in such physical characteristics as texture, i. e. ease of dispersibility, gloss and opacity necessary for commercial use in the paint and plastics industries. It should be noted that other titania-based yellow pigment compositions are disclosed in the reference, but they contain such additional metals as antimony, bismuth, uranium, tantalum copper, columbium (niobium), iron, arsenic, aluminum, lithium and magnesium. In addition, pigment compositions based on a non-titania host lattice are shown. A number of metals are specifically excluded, including cerium. It is desirable to produce an antimony-free pigment which also has the necessary texture, gloss and opacity for commercial use in the paint and plastics industries.
In addition to other factors, color intensity of pigments is directly proportional to calcination temperature and time. However, elevated temperatures promote sintering, an adverse process whereby the solid particles present become smooth, adhere to one another and densify to form large hard aggregates unsuitable for use as pigments in the paint and plastics industries. Thus, it is desirable to produce antimony-free pigments having the above desired properties while being relatively unsusceptable to sintering during calcination.